This invention relates to an improved process using a novel polyfunctional anionic initiator for the preparation of thermoplastic elastomeric block copolymers having the general formula A-B-(A).sub.n, in which A is a non-elastomeric polymer block with a glass transition temperature above 25.degree.C, B is an elastomeric polymer block with a glass transition temperature below 10.degree.C, and n denotes a whole or a fractional number, the process including polymerizing a monomer or monomer mixture to form the elastomeric polymer block B and, subsequently, polymerizing a monomer or monomer mixture to form the non-elastomeric polymer blocks A, the process conducted with the aid of a polyfunctional anionic initiator prepared by reacting a monofunctional anionic initiator with a monovinyl aromatic compound, a conjugated diene or both and thereafter with a polyvinylaromatic compound.
Block copolymers having the general formula A-B-(A).sub.n, where A is a non-elastomeric polymer block and B an elastomeric polymer block and n a whole or a fractional number have long been known. If the molecular weights of the non-elastomeric polymer blocks A are chosen between 200 and 100,000 and these of the elastomeric polymer block B between 20,000 and 1,000,000, these block copolymers as such possess elastomeric properties without being vulcanized. This is in contrast with other synthetic elastomers and natural rubber which can only be used as elastomers after being vulcanized.
An additional advantage of these block copolymers as regards processing to molded products is that owing to the elastomeric behavior of non-vulcanized block copolymers the amount of scrap formed is negligible. In addition, the block copolymers can be processed by the methods normally employed primarily with thermoplastic materials, such as extrusion and injection moulding.
The preparation of A-B-(A).sub.n block copolymers can be carried out according to known processes with both monofunctional and polyfunctional anionic initiators. Use of a mono- or bifunctional anionic initiator in the preparation of the A-B-(A).sub.n block copolymer yields a linear block copolymer comprising an elastomeric polymer block B carrying a non-elastomeric polymer block A on either side. If the A-B-(A).sub.n block copolymers are prepared with an anionic initiator having a functionally greater than two, star-shaped block copolymers will be obtained which are composed of a star-shaped elastomeric polymer block B, each branch of which carries a non-elastomeric polymer block A in the end or terminal position. The number of branches of the star-shaped polymer block B and the number of non-elastomeric polymer blocks A then theoretically equals the functionality of the initiator used and, as a consequence is a whole number.
In practice, however, many polyfunctional anionic initiators consist of mixtures of initiators differing in functionality, so that, on the average, the number of non-elastomeric polymer blocks may also be a fractional number.
A method of preparation using a monofunctional anionic initiator normally comprises three process steps, such as either successive polymerization of monomers to the three building polymer blocks of the A-B-A block copolymer, or successive polymerization of monomers to the A-B block copolymer plus subsequent coupling with a suited coupling agent to form the A-B-A- block copolymer.
On the other hand, a method of preparation using a polyfunctional anionic initiator would comprise only two reaction steps, namely polymerization to form the polyfunctional elastomeric polymer block B and subsequent polymerization of the polymer blocks A to form the block copolymer A-B-A. In special cases it might even be possible to perform the whole polymerization in one single process step.
Considered from the point of view of process engineering, application of polyfunctional anionic initiator in the preparation would therefore present a considerable advantage, especially having regard to equipment savings and favorable time econimics. A polyfunctional anionic initiator needs less reaction time for the block copolymerization which is appreciably shorter than when a monofunctional anionic initiator is employed.
The preparation of many polyfunctional anionic initiators presently known requires the presence of one or more polar solvents. After the preparation of the initiators has been completed, the polar solvents cannot, as a rule, be isolated and removed from the initiator to such a degree as to obviate interference with the polymerization of conjugated dienes. This is because even small amounts of polar solvents give rise to a sharp reduction of the cis-1,4 content and, as a consequence, detract considerably from the physical and mechanical properties of the resulting polymers. This explains why use of such polyfunctional anionic initiators, having even a small amount of polar solvent present therewith, in the preparation of block copolymers having the general formula A-B-(A).sub.n in which the elastomeric polymer block is normally composed of conjugated dienes, does not lead to optimum results.
On the other hand, there exist some polyfunctional initiators which can be prepared in the absence of polar solvents, but the results achieved with these initiators in the preparation of A-B-A block copolymers are usually not optimum. Examples of such intiators have been described in the German patent application No. 2,003,384 laid open to public inspection, the disclosure of which is hereby incorporated by reference. This patent application discloses that polyfunctional initiators can be prepared by allowing a monofunctional lithium-carrying anionic initiator to react with a polyvinyl aromatic compound. To prevent precipitate formation one or several monomers, such as butadiene or styrene, may be used in the preparation of such initiators. According to this above-mentioned patent application, however, the preparation is preferably carried out without such monomers being present. If, nevertheless, the butadiene and styrene monomers are employed, they may be added either before the reaction between the polyvinyl aromatic compound and the monofunctional anionic initiator, or after it. According to this above-mentioned patent application, the polyfunctional anionic initiators thus made may also be employed in the production of elastomeric block copolymers.
However, using the polyfunctional anionic initiators prepared according to the above-mentioned patent application applicants have not succeeded, even after numerous experiments, in producing block copolymers of the configuration A-B-(A).sub.n that can be employed as elastomers without previous vulcanization.